The invention relates generally to methods and apparatus for controlling an assignment of receiver fingers in a spread spectrum communication system, and more particularly to methods and apparatus for controlling assignment of receiver fingers in a multi-channel spread spectrum communication system.
Wireless spread spectrum communication systems are well known. For example, in single channel spread spectrum systems, a mobile station, such as a handheld telephone, Internet appliance, laptop computer or any other suitable device may communicate with one or more base stations over a wireless air interface. Typically, a mobile station is tuned to a single channel, such as a channel that communicates voice information. Multiple mobile stations may be tuned to the same frequency but may be assigned different spreading codes, can be differentiated by time (PN offset) or other suitable differentiation criteria. A receiver, that may be in a base station or in a mobile station, typically includes a plurality of receiver channel elements, such as a plurality of RAKE receiver fingers. Each finger of a RAKE receiver is typically assigned to a different traffic channel (e.g., spreading code). A searcher typically searches a pilot channel for energy peaks to provide receiver channel element management, also known as finger management. For example, the IS95 standard known as TIA/EIA-95-B Mobile Station-Base Station Compatibility Standard for Dual-Mode Spread Spectrum Systems. utilizes pilot power that is detected by a searcher to assign different fingers to receive information during different timing windows. If pilot energy is strong enough, it is assumed that the associated traffic channel is also strong enough. Once suitably adjusted, the energy received by each of the rake fingers is suitably combined and passed to a digital demodulator, such as a Viterbi decoder. Such conventional finger management systems typically use the energy per chip (Ec) per incident noise power (Io) to determine whether the searcher has detected a suitable energy peak at a particular point in time. Accordingly, the searcher provides the peak energy metrics to a finger management algorithm which then provides a receiver finger control signal to adjust a receiver finger to center about a selected window in time. If the searcher determines that the pilot energies are above an allowable threshold, then the finger management system determines if a receiver finger should be assigned to the requisite path or channel. Commonly, each receiver finger is assigned to a base station. Each base station typically has one or more different assigned Walsh codes for forward link (base-to-mobile) transmissions.
A problem arises in systems with multiple simultaneous communication link capabilities such as that described in 3GPP2 C.S0002-A (CDMA 2000 System) where a mobile station may be receiving multiple channels wherein each of the channels requires a different quality of service. For example, a mobile unit may simultaneously require a voice channel at one rate and a data channel at a different rate to provide differing services for a user. Accordingly, a mobile station is forced to manage the radio frequency environment for each channel or service while maintaining a radio frequency link performance that is appropriate for each type of channel. Optimization of received channel energy becomes increasingly important as additional channels are required. Unlike single channel systems, multi-channel systems may have a varying ratio of pilot energy to traffic channel energy since they may use a closed loop transmit power control scheme. The downlink (or uplink) power can be adjusted hundreds of times per second. Accordingly, pilot energy measurements may no longer be an accurate representation of traffic channel energies due to the rapid energy changes induced by fast closed loop power control. For example, a mobile station may send a transmit power control command to a base station every 1.25 milliseconds, thereby requiring a base station, for example, to increase to decrease power of a particular channel very often. In addition, multiple channels are simultaneously active by a mobile so that multiple voice channels may be operational as well as multiple data channels. In addition, multi-channel spread spectrum systems allow the higher rate channels, such as data channels, to be turned on and off during a call. Accordingly, traffic channel energies can vary drastically on a per user basis. Amobile station may be assigned multiple codes or different length codes, so that multiple channels need to be suitably received by a plurality of receiver channel elements, such as RAKE receiver fingers. Accordingly, in multiple channel systems, both a mobile station and a base station can create transmit control information such as power control bits (PCB""s) and tell each other to increase or decrease traffic channel power on a very rapid basis. Therefore, total pilot energy may not be a sufficient indicator of traffic channel energy. With more channels operating simultaneously in the rapidly changing channel power and different channel rates, determining receiver finger assignments can be quite difficult.
Accordingly, a need exists for a method and apparatus for controlling a plurality of receiver channel elements in a spread spectrum communication system.